Medical Device Link.

MATERIALS

Burgeoning market

Table I. (click to enlarge) Textile fibres used in medicine.

In each application area of biomedical textiles, advances continue, but they all depend on the properties of the fibre (or fibre blend) and the constructions fabricated from them. Biomedical textile fibres (Table I) comprise synthetic fibres and those derived from natural sources. Developments provide superabsorbent fibres, which are normally constructed from acrylic copolymers, with the ability to absorb up to 50 times their own weight in water. This property renders them useful for incontinence products and for inclusion in dressings for wounds that release large volumes of exudates. Increasingly important are resorbable fibres, which are specially developed to retain their mechanical properties in vivo for a specified period. The most widely used resorbable fibres are polylactic acid and polyglycolic acid and their copolymers. Others include polycaprolactone copolymers and polydioxanone and its copolymers. These materials can be specifically designed to function for a predetermined period of weeks or months, before undergoing degradation.

The fibres maintain their properties until they become significantly degraded through hydrolysis. The degradation kinetics are influenced by the chemistry of the hydrolysis process and by the penetration of water into the fibre. Thus, the pattern of degradation is governed, in practice, by a number of factors, which include the type of polymer or copolymer and the nature and temperature of the surrounding tissue. Although the rate of degradation will depend on the type of polymer or copolymer used, there is now increasing awareness of the influence of the fibre processing conditions that are employed. These include the fibre extrusion processing settings and the subsequent fibre drawing (stretching) regime. These, in turn, affect the geometry and internal structure of the fibre, factors that also govern the rate of penetration of water. The degradation products must of course be harmless to the body and there is concern that some toxic products may result from the breakdown of some copolymers.¹

Resorbable fibres used, for example, in sutures, offer the advantage that, because they progressively degrade as healing takes place after surgery, they do not need to be removed by further surgery. Scaffolds for tissue regeneration is another application area.

Textile fabric constructions

Figure 1. Network of artificial arteries.

Many different types of fabric construction are used (Table II) and the type influences the dimensional stability of the fabric, its porosity, its strength in particular directions and its ability to conform to different three-dimensional shapes. The major attraction of textiles for biomedical applications is the large number of different types of fibre from which fabrics can be constructed, and the versatility of the machinery available to produce a diversity of fabrics. In Figure 1, the textile component in this network of artificial arteries is constructed from warp-knitted polyester fibres.

Table II. (click to enlarge) Textile constructions for biomedical applications.

Table II provides examples of the applications for different types of textile construction. In general, woven fabrics are dimensionally the most stable, although for implants, they may fray if cut during surgery, with a risk of consequent device failure. Of the knitted fabrics, warp-knitted fabrics, in which the threads are fed along the direction of fabric production, are more extensively used. Their constructions tend to be more stable and versatile. Braided fabrics are produced through the interlacing of three or more threads, so that they can cross one another in a diagonal formation. The fabrics are flat or tubular. Nonwoven fabrics consist of webs of unoriented filaments or fibres and many are disposable.

Figure 2. Embroidered device for patient requiring extensive reconstruction to the shoulder.

Embroidered textile constructions are interesting. They involve the stitching of a needlework pattern onto a base fabric, which is subsequently removed by dissolution. The base fabric is normally polyvinyl alcohol, which is dissolved in water to leave behind the embroidered construction, which is often made of polyester fibres. If designed correctly, the embroidered structure that remains will hold together and be stable dimensionally. Embroidered structures can be readily customised to meet an individual patient’s needs. Figure 2 shows an embroidered construction made of polyester suture thread employed for shoulder reconstruction.

Recent developments

Surface modification. One important development in textile technology is the capability to modify the surface properties of textiles on a commercial basis using gas plasma treatments.² Gas plasmas consist of a mixture of positive and negative ions, electrons, free radicals, ultraviolet radiation and many different electronically excited molecules. The effectiveness of gas plasma treatments depends on the type of gas, the type of textile, the pressure within the plasma chamber, the frequency and power of the electrical supply, and the temperature and duration of the treatment.

These treatments, unlike traditional coating techniques, do not require solvents and thus are more appealing for biomedical applications. Textile surfaces can be made more biocompatible. The hydrophobic surfaces of polypropylene textiles can be rendered hydrophilic by treatment with oxygen plasma for in vivo applications. The interior wall of an artificial artery can be constructed from fabric treated with fluorinated gas plasma to reduce the likelihood of blood platelets adhering to the arterial wall and blood circulation being hindered.

Tissue-engineering scaffolds. Textiles are playing an increasing role in the construction of scaffolds for tissue engineering. The scaffold has to be constructed so that cells required for tissue regeneration or replacement in vivo or in vitro can adhere to the scaffold and multiply, and nutrients can readily reach the cells and waste carried away from them. Thus, the surface character of the scaffold and its porous nature are critical to its efficacy. The surface character can be modified by exposure to suitable gas plasma. The porous nature is governed by the construction of the textile fabric. Woven structures possess pores of regular size and shape. Nonwoven fabrics possess pores with wide variations in porous nature. Warp-knitted structures are used where high elastic deformation is required, for example, for reinforcing an organ subjected continuously to dynamic stress. Techniques are now available for promoting cell growth in particular directions. Some nonwoven fabrics can be carded (combed) so that many of the constituent fibres and the pores between them are oriented in the same direction. Oriented cell growth is thus encouraged. The development of fluted fibres is likely to have a similar effect: cells would grow along the channels of these fibres.

Permanent scaffolds. In some cases, after a surgical operation, there is a need for a permanent textile scaffold, usually to provide reinforcement after a wound has healed with full regeneration of cells. These scaffolds may be constructed from a range of synthetic fibres. In many applications, however, the scaffold becomes redundant and is then best removed. These scaffolds are increasingly being constructed from resorbable fibres so that surgery for the removal of the scaffold is rendered unnecessary. Moreover, continuing advances in controlling the active lifetimes of these fibres in vivo and in understanding the most suitable constructions for the repair of a particular type of tissue will greatly enhance surgical procedures.

Processing

The processing of fibres with cross-sections of the order of 100 nm or less is beginning to have an impact on medical textile constructions, especially on the construction of scaffolds for tissue engineering. These nanofibres can be produced by electrospinning. A polymer melt or solution is exposed to an electrostatic field of the order of 5–30 kV, which accelerates fine jets of the liquid polymer to an earthed target. On cooling or losing solvent, the fine jets form a web of fine fibres. Controlling the construction of the web could lead immediately to scaffolds of required designs.

Future developments

Auxetic fibres. An exciting development in fibre technology is the advent of auxetic fibres, which, in contrast to conventional fibres, swell on stretching, with consequent increase in their internal void volume. The processing of auxetic fibres from polymers such as polytetrafluoroethylene, polypropylene and nylon into knitted and woven textile constructions has been demonstrated by staff at Bolton University, UK.³ One application envisaged for auxetic fabrics is in wound bandages that contain a wound-healing agent. As the infected wound swells, so does the auxetic bandage. The internal voids in the bandage expand and release the wound-healing agent. Once the wound starts to heal, the swelling goes down, the bandage contracts, and release of the wound-healing agent ceases. Thus, the auxetic fibres provide a means of controlled drug delivery. Auxetic fabrics are also envisaged in compression bandages and arterial prostheses.

Shape-memory. Textiles constructed from shape-memory polymers⁴ are likely to also be increasingly significant in future for sutures and stents. Shape-memory materials “remember” a permanent shape as well as their current, temporary shape. Transfer from the temporary to the permanent shape is triggered by an external stimulus such as an increase in temperature. Thus, shape-memory implants can be brought into the body in a compressed temporary shape, through a small incision. A suitably constructed implant, on reaching body temperature, would then change to its “remembered” permanent shape. These materials can also be biodegradable, so that repeat surgery for removal of the implant would not be required.

Electronics. The incorporation of miniature electronic devices into textiles, which is already employed in protective clothing industries, will affect medical textiles. The use of electronic devices in textile implants is envisaged, for example, as monitors in artificial arteries, stents and heart valves constructed from textiles. When the implant begins to not function properly, they would act as warning devices and trigger electrical pulses or the release of drugs to overcome the problem, at least temporarily.

Controlled drug delivery. Textiles are likely to play a prominent role in controlled drug delivery at specific sites. The prospects for auxetic textiles in this respect have already been mentioned. Drugs present as additives in resorbable fabrics would gradually be released on breakdown of the fabrics. Another intriguing innovation is the development of soluble glass fibres for the controlled release of drugs in a wide range of concentrations and delivery periods.⁵

Future successes in biomedical textiles will reflect the seemingly infinite variety of textiles that can now be constructed, including bespoke constructions tailored in less than one day, and the nature of their constituent fibres. The future of textiles as medical devices, therefore, is not merely assured, but is burgeoning.

Dr Robert R. Mather is Director of Mather Technology Solutions, Upland House, Ettrick Road, Selkirk TD7 5AJ, UK, e-mail: mather@laburnum44.fsnet.co.uk